tree: don't use dataclass for numerous TreeEntry and Change

[efiop]

Same as #56

[codecov-commenter]

Codecov Report

Merging #59 (30df214) into main (cbebdcc) will decrease coverage by 0.04%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main      #59      +/-   ##
==========================================
- Coverage   34.28%   34.24%   -0.05%     
==========================================
  Files          33       33              
  Lines        1677     1679       +2     
  Branches      262      262              
==========================================
  Hits          575      575              
- Misses       1090     1092       +2     
  Partials       12       12              

Impacted Files	Coverage Δ
src/dvc_data/diff.py	0.00% <0.00%> (ø)

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